feat: infer reuse annotate --contributor value(s) from git

[szymonmaszke]

As in the title, currently I have the following alias/command defined (omitted other flags and arguments):

> reuse annotate --contributor="$(git config get user.name) <$(git config get user.email)>" 

It works okay, but has a few downsides, e.g.:

• Not easily usable within other tools (e.g. pdm scripts as defined here)
• Low visibility to users and feels somewhat hackish

One could implement by providing an additional boolean flag (let's say --contributor-from-git) and simply using subprocess.run(["git", "config", "user.name"] to avoid external dependencies.